The present invention relates to valve apparatuses and, particularly, to a valve apparatus comprising a flow control valve which is connected in pilot lines connecting a pilot valve and a pilot operated directional control valve to each other, for adjusting a switching speed of the pilot operated directional control valve.
U.S. Patent Application Serial No. 904,119, now U.S. Pat. No. 4,753,158 (which corresponds to EPC Patent Application No. 86112328.9 or Chinese Patent Application No. 86106036) discloses a pilot hydraulic circuit system in which a flow control valve has a restriction and a check valve connected to pilot lines connecting a pilot valve supplied with a pilot hydraulic fluid of a predetermined pressure set by a relief valve. A pilot operated directional control valve is actuated in response to operation of the pilot valve to permit free flow of the hydraulic fluid from the pilot valve to the directional control valve but also to restrict flow of the hydraulic fluid from the directional control valve to the pilot valve. Preferably, the flow control valve has added thereto a pressure compensating function making differential pressure across the restriction constant, so that the flow control valve is brought to a pressure-compensated flow control valve.
In general, in the conventional pilot hydraulic circuit system, when an operating lever is operated to return the pilot valve from an operating position to a neutral position, a return speed at which the directional control valve is returned to the neutral position is high. Therefore, a rise of in brake pressure generated in a main line becomes also extremely steep, so that large shock occurs on a hydraulic actuator which is controlled by the directional control valve.
In the pilot hydraulic circuit system disclosed in the above-mentioned patent application, by the action of the flow control valve, the hydraulic fluid flow is about to free flow when the pilot hydraulic fluid is supplied from the pilot valve to a pilot chamber of the directional control valve, but the check valve and the restriction function when the pilot hydraulic fluid is returned from the pilot chamber of the directional control valve to the pilot valve to restrict the flow of the hydraulic fluid. Also at this time, the flow rate of the restricted hydraulic fluid is pressure compensated so that the flow rate is maintained substantially constant. This causes the switching speed of the directional control valve to be adjusted to thereby eliminate the above-mentioned problem that the shock occurs on the hydraulic actuator.
For the flow control valve constructed as described above, the diameter of the restriction incorporated in the flow control valve must be made small because the flow rate of the pilot hydraulic fluid is low. In the case where the diameter of the restriction is small, however, the viscosity of the hydraulic fluid increases when the environmental temperature is low, and pressure loss of the hydraulic fluid passing through the restriction is increased so that the hydraulic fluid flows only at a flow rate lower than a set flow rate. In other words, the operating characteristic at low temperature deteriorates. This results in a difference in the switching speed of the directional control valve from season to season, in particular, between the summer season and the winter season, reducing stability of the operations of the hydraulic actuator.